my dad in law always had an Eagle Ultra Classic portable fish finder. he would put it in the aluminum boat when on vacation and the boats were not his. They are still on ebay for $50. Only downside is that they took like 8 D cell batteries which costs a fortune these days. I rigged up a lead acid 12v battery that would fit in the battery case (like in a security camera) and that gave us at least the chance to recharge it every night.

I'm sure there are newer models with better graphics, more compact, consume less power since these are popular for ice fishing.